Problems with sending SC attachments to backend

I have configured it as stated in the self service procurement configuration guide.

In BAdI BBP_CREATE_REQ_BACK (we always create purchase requisitions) I set CV_ATTACH_TRANSFER_ACTIVE = 'X'. The implementation and its method is active.

In R/3 backend I created a new document type SRM using transaction DC10, then set the description to "SRM Documents" (in case this matters) and also set the "Use KPro" indicator... etc. everything according to the config guide.

Whatever I tried until now, it doesn't work. I think it must be something related to DMS customizing. We didn't use it previously, so it wasn't customized at all before that. Is there more to do in R/3 backend customizing in order to achieve this?